MaybeUninit::write_slice_cloned() makes it very easy to accidentally leak

[mgeier]

It is expected behavior that objects assigned/written to MaybeUninit might leak at some point (if assume_init() or similar is not used). However, MaybeUninit::write_slice_cloned() returns a &mut [T] (which might be passed on to further functions, where its origin might not be apparent), and objects assigned to that might leak as well, which I think is less obvious.

playground example

I don't know if there is a way to improve the API to mitigate this problem, but I think at least the documentation should point out the potential leaking.

Currently, it is noted that already initialized values in MaybeUnint will not be dropped:

rust/library/core/src/mem/maybe_uninit.rs

Line 923 in 780b094

/// Any already initalized elements will not be dropped.

However, it is not mentioned that assigning to the resulting slice might also lead to leaks.

MaybeUninit::write_slice_cloned() was added in #79607
Tracking issue: #79995
See also https://users.rust-lang.org/t/is-there-a-way-to-copy-t-into-mut-maybeuninit-t-without-unsafe/51301

[beepster4096]

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